1. Field of the Invention
This invention relates to an intervertebral disc prosthesis and to surgical techniques for implanting the prosthesis.
The vertebrae of the spinal column are connected together by intervertebral fibrocartilaginous discs. The discs maintain a separation between the vertebrae, but occasionally become narrowed so that the intervertebral separation is reduced. This reduction in separation has a number of painful and unpleasant consequences, for example, in the cervical region of the spine, it can result in cervical spondylosis, vertebral artery syndrome and painful arc syndromes. These symptoms are described in more detail below.
1. Description of the Prior Art
Procedures have been developed in the prior art for alleviating the symptoms resulting from intervertebral disc failure.
One such procedure involves fusing the adjacent vertebrae by removing the damaged disc and inserting a plug or wedge of bone removed from another part of the patient's skeleton.
An example (known as the "Cloward technique") of such a procedure for the cervical vertebrae is described in "ORTHOPAEDICS--PRINCIPLES AND THEIR APPLICATION," Samuel L. Turek, M.D., Lippincott Company, Third Edition, pp. 761-763, in which a hole is drilled in the spinal column straddling the damaged disc space and including parts of the adjacent vertebrae. The hole is then filled with a cylindrical plug or dowel of bone in order to fuse the vertebrae together.
Fusion of vertebrae together necessarily results in complete loss of flexibility of the spinal column at this location and is thus disadvantageous. Accordingly, proposals have been made in the past to replace the damaged or diseased disc with a disc prosthesis intended to duplicate the function of the natural disc to some extent.
French patent application publication No. 2,372,622, of Bernard Fassio, published June 30, 1978, discloses one such disc prosthesis. This consists of a flat circular plate having central hemispherical projections on each face thereof. The hemispheres allow articulation of the joint while the flat plate maintains separation. It is believed, however, that such a prosthesis would not be entirely satisfactory because the hemispheres would not correspond closely to the shape of the adjacent vertebral surfaces. This could result in crushing of the cancellous vertebral bone by the hemispheres and consequent reduction in separation and articulation of the joint.
Another disc prosthesis is disclosed in U.S. Pat. No. 3,867,728 of Stubstad et al., issued Feb. 25, 1975, assigned to Cutter Laboratories Inc. The prosthesis is a flattened kidney shaped block of elastomeric synthetic resin. The shape of the prosthesis is intended to conform closely to the space in a spinal disc from which the necleus pulposus has been removed. One disadvantage of this type of prosthesis is that elastomeric materials have been known to disintegrate in the body and to break down under repeated stressing over prolonged periods. Moreover, it is disclosed that the surface of the prosthesis may be porous to allow tissue ingrowth, but a porous surface is more prone to harbor bacteria due to the large surface area involved. In a porous surface in which tissue ingrowth has occurred, there is also increased possibility of repeated injury at the interface between the bone and prosthesis due to tearing of fibrous tissue or prosthetic fibers with subsequent tissue reaction including foreign body rejection.
A major disadvantage of a porous material is that if there is actual tissue ingrowth into the prosthesis, removal could be difficult. Curetting out the prosthesis would lead to hemorrhage of a very vascular surface area, caused by the tissue ingrowth, with subsequent increased danger of cord compression secondary to hemorrhage.
Besides, any porous material allowing tissue ingrowth for stability in a cervical spine would be dangerous as the esophagus, which lies anteriorly, could become adhered to the prosthesis with resultant dysphagia or difficulty in swallowing.
There is consequently a need for a prosthesis that remains stably in place when implanted but does not have the difficulties referred to above associated with porous surfaces.
West German Offenlegungsschrift No. 2,263,842 in the name of Hoffmann-Daimler, published on July 4, 1974 discloses yet another type of disc prosthesis. In its simplest form, this prosthesis consists of a circular disc having smooth convex faces. The disc may be made of a synthetic material. This simple form, however, may become displaced when implanted and could possibly damage the neural canal.
The known prostheses are often made of different materials bonded together to allow movement within the prosthesis, but this weakens the overall loading strength of the prosthesis. This is particularly the case when the prosthesis is submitted to repeated stresses. Bonded materials can become fatigued under stress and, due to the very thin spaces involved, any multiple layered prosthesis would involve quite thin layers of material. This would increase the problem of breakage and migration of fragments which could have hazardous consequences in the spinal area.
It is believed that each of the prostheses referred to above is intended only for the lumbar area of the spine. For example, the Stubstad et al. patent describes insertion through an anterior approach which is, in fact, a `retroperitoneal` approach. There is therefore a need for a disc prosthesis that can be used in the cervical area of the spine as well as just the lumbar area.
Another difficulty of the known prostheses is the difficulty of handling them during surgical implantation. In particular, difficulty is often encountered in removing the prostheses once implanted should adjustment or replacement be required.